This chapter introduces the concept of social milieus that studies social units on the basis of shared sociocultural characteristics and does not presuppose a similar income. The notion “middle class” implies more than being part of a certain income range and it contains connotations about related values, political orientations and lifestyles. The United Nations formulate, for instance, the growth of the “middle classes” as part of their first goal, the eradication of poverty and hunger. One important conceptual distinction that clarifies differences between structural and sociocultural analysis, is the contrast between the approaches of “middle stratum” or “middle-income stratum” and “middle class”. The chapter shows by examples from Brazil and Kenya how diverse the economic and sociocultural differentiation of middle-income strata in two countries of the Global South is, and by which concepts it is possible to consider the particularities. It summarizes classical approaches to inequality from economics and sociology.